Field of the Invention
The present invention relates to a dielectric material for a multilayer ceramic capacitor (MLCC), as well as a multilayer ceramic capacitor using said dielectric material.
Description of the Related Art
Dielectric materials have been developed with the aim of making multilayer ceramic capacitors (MLCC) smaller in size, while increasing their capacitance. Dielectric materials for small, large-capacity multilayer ceramic capacitors are constituted by barium titanate, or BaTiO3, and small amounts of additives. Recent years have seen a trend for accelerated sintering at lower temperature, which makes it difficult for the additives to diffuse and make them solid-solubilized fully during sintering, and this in turn makes it difficult to achieve the intended fine structure. A highly dispersed state of additives is desired as a solution, and materials constituted by BaTiO3 in which additives are pre-solid-solubilized are being developed as a means to achieve this highly dispersed state of additives.
Patent Literature 1 discloses wet-reacting a titanium compound, barium compound, and compounds containing secondary constituent elements, and then calcinating the obtained product to prepare a perovskite composite oxide constituted by barium titanate grains in which the secondary constituent elements are solid-solubilized, and using this perovskite composite oxide, along with polymeric materials, to produce a composite dielectric material.
The distributions of abundance ratios of the aforementioned donor elements and other additive elements in the ceramic grains constituting the dielectric layers also affect the performance of the multilayer ceramic capacitor. Regarding this point, Patent Literature 2, for example, describes a dielectric ceramic with improved dielectric breakdown voltage, whose additive elements such as Mn, V, Cr, Co, Ni, Fe, Nb, Mo, Ta, and W are distributed roughly uniformly over the entire area from the crystal grain boundary to the center. In an example cited in this literature, barium carbonate, titanium oxide, and oxides of the aforementioned additive elements were mixed and calcinated at 1200° C., after which other compounds of additive elements were added and the mixture was calcinated further at 1000° C. in an oxidizing ambience, after which the obtained ceramic material mixture was used to prepare green sheets that were stacked and sintered for 2 hours at 1200° C. in a reducing ambience, and then heat-treated for 30 minutes at 600° C. in an oxidizing ambience, to obtain a laminated capacitor. The literature describes that, although the distributions of the additive elements in the dielectric ceramic constituting the laminated capacitor thus obtained were regulated to be roughly uniform, as mentioned above, actually, there were about a seven-fold difference between the grain boundary area and the center area.
Patent Literature 3 proposes a multilayer ceramic capacitor whose service life does not decrease due to dielectric breakdown, etc., even when its dielectric layers are increased and/or made thinner, thus allowing for size reduction and capacitance increase, wherein said multilayer ceramic capacitor is made of ceramic grains that are each constituted by a crystalline core and a shell that surrounds the core, where additive elements such as Mn, V, Cr, Mo, Fe, Ni, Cu, and Co are added to the core and the concentrations of these additive elements increase from the center of the core toward the shell. In an example cited in this literature, barium carbonate, titanium oxide, and chemical compounds of the aforementioned additive elements were mixed and calcinated for 2 hours at 200° C. to synthesize a barium titanate containing the additive elements, after which other of additive elements were added and the mixture was calcinated for 2 hours at 1000° C. to obtain ceramic grains, and these ceramic grains were used to prepare ceramic green sheets that were stacked and sintered for 3 hours at 1130° C. in a reducing ambience, and then heat-treated for 30 minutes at 600° C. in an oxidizing ambience, to obtain a multilayer ceramic capacitor. It is indicated that the concentrations of the additive elements in the cores, and concentrations of the additive elements in the shells, of the ceramic grains constituting the dielectric layers of the obtained multilayer ceramic capacitor, were approx. 290 ppm and approx. 410 ppm, respectively.
Also, Patent Literature 4 describes barium titanate ceramic grains, as a dielectric ceramic that gives a multilayer ceramic capacitor offering good capacitance vs. temperature characteristics as well as excellent service life characteristics, wherein said barium titanate ceramic grains are characterized in that they each have a core and a shell, and contain rare earth metals R and M (M is at least one type of element selected from a group that includes Mg, Mn, Ni, Co, Fe, Cr, Cu, Al, Mo, W, and V) as secondary constituents, where the total concentration of R and M slopes from the grain boundary toward the core and there are areas where the total concentrations becomes the minimum and maximum.